High fiber density, low bend loss optical fiber cable

ABSTRACT

A highly packed, low bend loss optical cable is provided. The cable includes an outer cable jacket and a plurality of buffer tubes surrounded by the cable jacket. Each buffer tube includes an inner surface defining a channel having a diameter, D 1 , and an outer surface facing an inner surface of the cable jacket. The cable includes a plural number, N, of optical fibers, located within the channel of each buffer tube and surrounded by the inner surface of the buffer tube. Each optical fiber has an outer diameter, D 2 . The N optical fibers are densely packed within each buffer tube such that a diameter ratio parameter, Ω, is defined as the ratio D 1 / D 2 , and is 2.25+0.143(N)≤Ω≤1.14+0.313(N).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 to U.S. Provisional Application No. 62/341,369, filed on May 25, 2016, and is incorporated herein by reference.

BACKGROUND

The disclosure relates generally to cables and more particularly to fiber optic cables having a high optical fiber density and low bend loss despite the high optical fiber density. Optical cables have seen increased use in a wide variety of fields including various electronics and telecommunications fields. Optical cables contain or surround one or more optical fibers. The cable provides structure and protection for the optical fibers within the cable.

SUMMARY

One embodiment of the disclosure relates to a densely packed, low bend loss optical fiber cable. The cable includes an outer cable jacket and a plurality of buffer tubes surrounded by the cable jacket. Each buffer tube includes an inner surface defining a channel having a diameter, D1, and an outer surface facing an inner surface of the cable jacket. The cable includes a plural number, N, of optical fibers, located within the channel of each buffer tube and surrounded by the inner surface of the buffer tube, and each optical fiber has an outer diameter, D2. The N optical fibers are densely packed within each buffer tube such that a diameter ratio parameter, C2, is defined as the ratio D1/D2, and is 2.25+0.143(N)≤Ω≤1.14+0.313(N). Each of the plural number of optical fibers includes a mode field diameter of larger than 9 microns at 1310 nm, a cable cutoff of less than 1260 nm and a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 15 mm.

An additional embodiment of the disclosure relates to a densely packed, low bend loss optical fiber cable. The cable includes an outer cable jacket and a plurality of buffer tubes surrounded by the cable jacket. The buffer tubes each include an inner surface defining a channel and an outer surface facing an inner surface of the cable jacket. The cable includes an inner group of at least one optical fiber located within the channel of each buffer tube and an outer group of a plurality of optical fibers located within the channel of each buffer tube and surrounding the inner group. The optical fibers of the outer group and inner group are packed within each buffer tube such that a maximum gap length measured between any pair of adjacent optical fibers of the outer group is less than an outer diameter of the at least one optical fiber of the inner group such that the at least one optical fiber of the inner group is blocked from moving from the inner group to the outer group. Each of the optical fibers of the inner group and outer group include a mode field diameter of larger than 9 microns at 1310 nm, a cable cutoff of less than 1260 nm and a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 15 mm.

An additional embodiment of the disclosure relates to an optical cable. The cable includes a polymeric outer cable jacket with outer diameter of less than 15 mm, a strength member surrounded by the outer cable jacket and a plurality of buffer tubes surrounding the strength member. The cable includes at least four optical fibers located within each buffer tube. Each optical fiber includes a mode field diameter of larger than 9 microns at 1310 nm, a cable cutoff of less than 1260 nm and a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 15 mm.

An additional embodiment of the disclosure relates to an optical cable. The optical cable includes a polymeric outer cable jacket with an outer diameter of less than 15 mm, a buffer tube surrounded by the outer cable jacket and a plurality of optical fiber ribbons arranged in a stack within the buffer tube. Each of the optical fiber ribbons includes a plurality of optical fibers surrounded by and supported within a polymeric ribbon matrix. Each optical fiber includes a mode field diameter of larger than 9 microns at 1310 nm, a cable cutoff of less than 1260 nm and a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 15 mm.

An additional embodiment of the disclosure relates to a densely packed, low bend loss optical fiber cable. The cable includes an outer cable jacket and a plurality of buffer tubes surrounded by the cable jacket. Each buffer tube includes an inner surface defining a channel having a diameter, D1, and an outer surface facing an inner surface of the cable jacket. The cable includes a plural number, N, of optical fibers, located within the channel of each buffer tube and surrounded by the inner surface of the buffer tube, and each optical fiber has an outer diameter, D2. The N optical fibers are densely packed within each buffer tube such that a diameter ratio parameter, Ω, is defined as the ratio D1/D2, and is 2.25+0.143(N)≤Ω≤2.66+0.134(N). Each of the plural number of optical fibers includes a mode field diameter of larger than 9 microns at 1310 nm, a cable cutoff of less than 1260 nm and a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 20 mm.

Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an optical fiber cable according to an exemplary embodiment.

FIGS. 2A-2C are cross-sectional views of optical fiber buffer tubes according to exemplary embodiments.

FIG. 3 is a graph showing the minimum ratio of buffer tube inner diameter to optical fiber outer diameter as a function of number of optical fibers in the buffer tube.

FIG. 4 is a cross-sectional view of an optical fiber cable according to another exemplary embodiment.

FIG. 5 is a cross-sectional view of an optical fiber cable according to another exemplary embodiment.

FIG. 6 is a cross-sectional view of an optical fiber cable according to another exemplary embodiment.

FIG. 7 is a cross-sectional view of an optical fiber cable according to another exemplary embodiment.

FIG. 8 shows a refractive index profile corresponding to one embodiment of an optical waveguide fiber as disclosed herein.

FIG. 9 shows a refractive index profile to an embodiment of an optical fiber as disclosed herein.

FIG. 10 shows a refractive index profile of an embodiment of an optical fiber as disclosed herein.

FIG. 11 shows a refractive index profile of an embodiment of an optical fiber as disclosed herein.

FIG. 12 shows a refractive index profile of an embodiment of an optical fiber as disclosed herein.

FIG. 13 shows a refractive index profile of an embodiment of an optical fiber as disclosed herein.

FIG. 14 shows a refractive index profile of an embodiment of an optical fiber as disclosed herein.

DETAILED DESCRIPTION

Referring generally to the figures, various embodiments of an optical fiber cable are shown. As background, some optical fiber cable applications are deployed or used in ways that may induce bend losses in optical signals transmitted through the optical fibers of the cable. Such bend losses can be caused by cable deployments that include tight bend radii, compression of optical fiber, etc., that induce bend losses. Further, such bend losses can be experienced in a wide variety of cables, such optical drop cable assemblies, distribution cables with Factory Installed Termination Systems (FITS) and slack loops.

In addition, bend losses may be greater in cables in which optical fibers are densely packed in relatively rigid buffer tubes, are densely packed within a cable jacket and/or are densely packed in an optical fiber ribbon arrangement. Bend losses in such cables are caused, at least in part, by positional constraint resulting from the dense packing which limits the ability of optical fibers to shift to assume low strain positions during bending, compression, etc. Thus, typical optical fiber cable and/or buffer tube configurations include a significant amount of free-space between the outer surfaces of the optical fibers and the inner surface of the buffer tube or jacket, and this free-space allows the optical fibers to move or shift to assume low stress positions during bending. By reducing stress that the optical fibers experience during bending, such low density cables provide a satisfactory level of signal attenuation, but do so with a relatively large diameter buffer tube and/or relatively large cable jacket.

In particular embodiments discussed herein, Applicant has developed a new highly packed buffer tube and highly packed cable design that utilizes a new small diameter, low bend loss optical fiber design. As discussed herein, Applicant has developed a highly packed, low diameter buffer tube having low bend loss, and similarly, Applicant has developed a highly packed, low diameter optical fiber cable having low bend loss. In particular embodiments, Applicant believes that the low diameter, bend insensitive optical fibers as discussed herein allow for the dense fiber packing, even though conventional buffer tube/cable design wisdom would indicate that such packing density would result in unacceptable bend losses due to the positional constraint and resulting increase stress on the optical fibers during bending. Such low diameter, highly packed fiber optic cables may provide a number of advantages including limiting duct congestion, allow for blowing of cables through ducts, reducing cable weight and material usage, etc.

Referring to FIG. 1, an optical cable, shown as cable 10, is illustrated according to an exemplary embodiment. Cable 10 includes an outer cable jacket, shown as outer jacket 12, having an inner surface 14 that defines an inner passage or cavity, shown as central bore 16, and an outer surface 18 that generally defines the outermost surface of cable 10. As will be generally understood, inner surface 14 of jacket 12 defines an internal area or region within which the various cable components discussed herein are located.

In various embodiments, cable jacket 12 is formed from an extruded thermoplastic material. In various embodiments, cable jacket 12 may be a variety of materials used in cable manufacturing such as polyethylene, medium density polyethylene, polyvinyl chloride (PVC), polyvinylidene difluoride (PVDF), nylon, polyester or polycarbonate and their copolymers. In addition, the material of cable jacket 12 may include small quantities of other materials or fillers that provide different properties to the material of cable jacket 12. For example, the material of cable jacket 12 may include materials that provide for coloring, UV/light blocking (e.g., carbon black), burn resistance, etc.

Cable 10 includes one or more optical transmission elements or optical waveguides, shown as optical fibers 20. In the embodiment shown, groups of optical fibers 20 are located in separate buffer tubes 22, and buffer tubes 22 are wrapped (e.g., in an SZ stranding pattern) around a central strength member 24. In various embodiments, cable 10 includes at least four buffer tubes 22. Central strength member 24 may be any suitable axial strength member, such as a glass-reinforced plastic rod, steel rod/wire, etc. Generally, cable 10 provides structure and protection to optical fibers 20 during and after installation (e.g., protection during handling, protection from elements, protection from the environment, protection from vermin, etc.).

In various embodiments, cable 10 also includes an armor layer, shown as armor 26. In general, armor 26 is formed from a strip of metal material (e.g., a metal tape, a flat elongate continuous piece of material, etc.) that is wrapped around and circumferentially surrounds buffer tubes 22. As shown in FIG. 1, armor 26 is located adjacent to the inner surface of outer jacket 12 such that these two layers are in contact with each other. In specific embodiments, armor 26 is corrugated steel tape material that is wrapped around the interior portions of cable 10, and in some such embodiments, armor 26 is longitudinally folded forming a longitudinal overlapped section where opposing edges of the tape overlap to completely surround inner buffer tubes 22 (and any other interior component of cable 10). In other embodiments, armor 26 may be a strip of metal tape material, helically wrapped around buffer tubes 22 such that armor 26 forms a layer circumferentially surrounding buffer tubes 22. In general, armor layer 26 provides an additional layer of protection to fibers 20 within cable 10, and may provide resistance against damage (e.g., damage caused by contact or compression during installation, damage from the elements, damage from rodents, etc.). Cable 10 may include a variety of other components or layers, such as helically wrapped binders, circumferential constrictive thin-film binders, water blocking tape materials, water-blocking fiber materials, etc. As defined herein the minimum cable core diameter is the minimum diameter without stranding of the buffer tube bundle surrounding and the central member. In some embodiments, stranding the buffer tubes will increase the diameter of the cable core by 1 to 15 percent. In some embodiments, stranding the buffer tubes will increase the diameter of the cable core by 1 to 5 percent.

In the embodiment shown, cable 10 includes one or more preferential tear feature and/or ripcord 28 embedded in or underneath jacket 12. In this embodiment, preferential tear feature and/or ripcord 28 is located with jacket 12 such that ripcord 28 facilitates opening of outer jacket 12. In some embodiments, ripcord 28 may be located within armor layer 26 such that ripcord 28 facilitates opening of both armor 26 and jacket 12.

As noted above, cable 10, optical fibers 20 and buffer tube 22 are configured in various ways to provide a high fiber density, high fiber count cable while at the same time reducing or minimizing buffer tube size and/or cable jacket size. As discussed herein, low diameter optical fiber allows higher density and smaller cable, and the low bend loss design of these optical fibers allows such high density and small cable to have acceptable signal loss properties.

Referring to FIGS. 2A-2C, various buffer tube designs having different levels of fiber packing density are shown. FIGS. 2A-2C show three different buffer tube designs, shown as buffer tubes 22′, 22″ and 22′″. In general, buffer tubes 22′, 22″ and 22′″ are polymeric tubes that surround, protect and organize optical fibers 20, and further buffer tubes 22′, 22″ and 22′″ are generally the same as each other except for the buffer tube's inner diameter and the resulting optical fiber packing arrangement within each buffer tube discussed in more detail below. Further, it should be understood that cable 10 may include buffer tubes 22′, 22″ and/or 22′″ in any combination. In various embodiments, cable 10 includes only one of buffer tubes types 22′, 22″ or 22′″, and in other embodiments, cable 10 includes a mixture of buffer tubes 22′, 22″ and/or 22′″.

Referring to FIG. 2A, buffer tube 22′ includes a buffer tube wall 30 having an inner surface 32 and an outer surface 34. Inner surface 32 defines a buffer tube channel 36 within which optical fibers 20 are located. As shown in FIG. 2A, optical fibers 20 are arranged into an outer group 38 and an inner group 40 within channel 36. Generally, the optical fibers 20 of outer group 38 are located in the outer portion of channel 36 adjacent to (e.g., less than 0.5 microns from) or in contact with inner surface 32 such that outer group 38 surrounds inner group 40. Inner group 40 is generally located in a central region of channel 36.

Inner surface 32 defines a buffer tube inner diameter D1, and in the specific embodiment shown in FIG. 2A, D1 is sized such that optical fibers 20 have full positional constraint. In the specific embodiment shown in FIG. 2A, the inner diameter of buffer tube 22′ is small relative to the outer fiber diameter, D2, and the number of fibers, N, which results in the full positional constraint shown in FIG. 2A. In particular, buffer tube 22′ is sized such that a maximum gap length, shown as G, measured between any pair of adjacent optical fibers 20 of outer group 38 is less than an outer diameter, D2, of one or more optical fiber 20 of inner group 40 such that optical fibers 20 of the inner group 40 are blocked from moving from the inner group 40 to the outer group 38. Thus, in this manner buffer tube 22′ results in a fully positionally constrained fiber arrangement, and in this arrangement, buffer tube 22′ provides a densely packed unit of optical fibers. Specifically, the portion of the area of channel 36 occupied by optical fibers 20 is high, and the overall inner diameter D1 is low resulting in buffer tube with a large fiber count in a relatively small area.

In particular embodiments, Applicant has determined a relationship between D1 and D2 that defines dense packing of optical fibers 20 within buffer tube 22′ as provided by the present disclosure. As a specific example, in various embodiments discussed herein, optical fibers 20 are densely packed within buffer tube 22′ such that a diameter ratio parameter, Ω (Omega), which is defined as the ratio, D1/D2, is less than 2.66+0.134(N), where N is the number of optical fibers 20 within buffer tubeb 22′, and in addition, in a specific embodiment, Ω is also greater than 2.25+0.143(N), where N is the number of optical fibers 20 within buffer tube 22′. In various embodiments, N is at least 4, specifically N is greater than 6, and more specifically N is 8 to 24, inclusive of 8 and 24. In another embodiment, N is 12 to 24, inclusive of 12 and 24. In another embodiment, N is greater than 24, and in one such embodiment, 8≤N≤48. In the specific embodiment shown in FIG. 2A, N is 12, D1 is greater than or equal to 4.030 times the fiber OD and less than or equal to 4.273 times the fiber OD. In addition, in this 12 fiber embodiment, outer group 38 has 9 optical fibers 20 and inner group 40 has 3 optical fibers 20.

In various embodiments, the degree of packing of optical fibers 20 within tube 22′ can be understood as the ratio of the minimum diameter, D4, of a circle circumscribing all fibers 20 of outer group 38. In the embodiments shown in FIGS. 2A-2C, D4 is relatively large such that more than half, and specifically all fibers 20 of outer group 38 are in contact with inner surface 32. In various embodiments, fibers 20 are packed such that D4/D1 is greater than 0.95, specifically is greater than 0.97, more specifically greater than 0.99 and even more specifically greater than 0.995. In various cable embodiments utilizing these densely packed buffer tubes, Applicant believes that the interaction between the outer surfaces of fibers 20 and the inner surface 32 of tube wall 30 may increase tensile strength of the cable constructed from such tubes, and in such embodiments, the utilization of the various bend insensitive fibers discussed herein provides for satisfactory optical attenuation despite the high level of fiber/tube interaction.

Referring to FIG. 2B and 2C, different levels of fiber packing density and positional constraint are shown. FIG. 2B shows a buffer tube 22″ sized to provide partial positional constraint to optical fiber 20 movement within buffer tube 22″, according to an exemplary embodiment. As shown in FIG. 2B, inner diameter D1 of buffer tube 22″ is sized such that one of the optical fibers 20 of inner group 40 just fits in the gap 42 between a pair of adjacent optical fibers 20 of outer group 38 and can move back into the inner group 40. As a comparison, FIG. 2C shows buffer tube 22′″ sized to provide no positional constraint on optical fibers 20 within buffer tube 22′″.

In various embodiments, buffer tubes 22 of cable 10 as discussed herein have at least some positional constraint. As a specific example, in various embodiments discussed herein, optical fibers 20 are densely packed within a buffer tube, such as buffer tube 22′ of 22″, such that a diameter ratio parameter, Ω, which is defined as the ratio, D1/D2, is less than 1.14+0.313(N), where N is the number of optical fibers 20 within buffer tube 22′ or 22″, and in addition, in a specific embodiment, Ω is also greater than 2.25+0.143(N), where N is the number of optical fibers 20 within buffer tube 22′ and 22″. In various embodiments, N is at least 4, specifically N is greater than 6, and more specifically N is 8 to 24, inclusive of 8 and 24. In another embodiment, N is 12 to 24, inclusive of 12 and 24. In another embodiment, N is greater than 24, and in one such embodiment, 8≤N≤48. In the specific embodiment shown in FIG. 2B, N is 12, D1 of buffer tube 22″ is greater than or equal to 4.273 times the fiber OD and less than or equal to 4.87 times the fiber OD and specifically less than 4.864 times the fiber OD. In addition, in this 12 fiber embodiment of buffer tube 22″, outer group 38 has 9 optical fibers 20, and inner group 40 has 3 optical fibers 20, and one optical fiber 20 of inner group 40 is permitted to move partially into gap 42 as shown in FIG. 2B.

As a comparison, FIG. 2C, shows buffer tube 22′″ sized to have no positional constraint such that one or more optical fibers 20 are permitted to move freely between inner group 40 and outer group 38, which allows optical fibers 20 to assume low strain positions during bending, but requires a larger inner diameter D1 and less dense packing as compared to buffer tubes 22′ and 22″. Specifically, as shown in FIG. 2C, optical fibers 20 are not densely packed within buffer tube 22′″ such that a diameter ratio parameter, Ω, of buffer tube 22′″ which is defined as the ratio, D1/D2, is greater than 1.14+0.313(N), where N is the number of optical fibers 20 within buffer tube 22″, for, N greater 6, N greater than 24, and N 8 to 24, inclusive of 8 and 24. In the specific embodiment shown in FIG. 2C, N is 12, D1 of buffer tube 22′ is greater than or equal to 4.864 times the fiber OD.

In specific embodiments, the dense fiber packing and high fiber count within buffer tube 22′ is facilitated by an optical fiber having a low outer diameter, and various optical fiber properties that allows for low signal loss despite the dense packing and high fiber count. In various embodiments, such fibers may have a variety of properties in various combinations, such as an outer diameter D2 less than 210 microns, a mode field diameter of larger than 9 microns at 1310 nm, a cable cutoff of less than 1260 nm, a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 20 mm, and/or a wire-mesh covered drum microbending loss at 1550 nm of less than 0.03 dB/km. In specific embodiments, optical fibers 20 may be any of the optical fibers or include any of the optical fiber features or characteristics discussed herein.

In various embodiments, tube walls 30 of buffer tubes 22′, 22″ and 22′″ are relatively rigid, relatively thick polymer structures such that during bending, tube walls 30 do not deform to a sufficient degree to allow for optical fibers 20 to assume a low strain position through deformation of the buffer tube wall itself. This is in contrast to some optical fiber bundles or micromodules that are densely packed by utilizing flexible, thin bundle jackets. In such optical fiber bundles dense packing and acceptable bend loss characteristics are achieved through the flexible nature of the bundle jacket which allows for fiber movement during bending. In contrast to such optical fiber bundles, in various embodiments, buffer tubes 22′ and 22″ do not allow for substantial movement and achieves acceptable bend loss characteristics via use of low diameter, bend insensitive optical fibers, such as those discussed herein.

In various embodiments, tube walls 30 have a thickness T1 that is between 50 microns and 250 microns. In particular embodiments, tube walls 30 are formed from a material having a having a modulus of elasticity at 25° C. between 0.8 GPa and 3 GPa. In various embodiments discussed herein, tube walls 30 having these thicknesses and/or moduli form relatively rigid tubular structures that do not bend, stretch, deform, etc. to a significant amount within the cable, and thus, in such embodiments, optical fibers 20 are bend insensitive fibers as discussed herein. Such bend insensitive fibers allow for low optical attenuation despite residing in highly packed, thick and/or high modulus tubes 30.

Buffer tube walls 30 may be made from a variety of suitable polymer materials. In one embodiment, buffer tube walls 30 are formed from a polypropylene material. In another embodiment, buffer tube walls 30 are formed from a polycarbonate material. In various embodiments, buffer tube walls 30 are formed from one or more polymer material including polybutylene terephthalate (PBT), polyamide (PA), polyoxymethylene (POM), polyvinylchloride (PVC), flame retardant PCV, poly (ethylene-co-tetrafluoroethene) (ETFE), or various combinations of the polymer materials discussed herein, etc. In various embodiments, the material of buffer tube walls 30 may include various fillers or additives including UV blocking materials and burn resistant materials.

As a specific example of the various buffer tube sizing and fiber packing of the present disclosure, FIG. 3 shows a plot of the minimum ratio of buffer tube inner diameter to fiber outer diameter as a function of the number of fibers in the buffer tube. Plot 50 shows the normalized effective bundle diameter assumed for a defined number of fibers in a bundle and plot 52 shows the minimum normalized tube ID to just accommodate a defined number of fibers. This is a fit to the modeled data defining the minimum normalized diameter of circumscribing circle to just fit around each bundle of fibers from 1 to 48 fibers.

Referring back to FIG. 1, in particular embodiments, one or more buffer tubes 22 of cable 10 is one or more of densely packed buffer tube, such as buffer tubes 22′ and 22″ discussed above. In such embodiments, the dense packing and small diameter of buffer tubes 22′ or 22″ allow for cable 10 to also be densely packed and have a small diameter despite having a large number of optical fibers 20. As shown in FIG. 1, cable 10 has an outer diameter, D3, that is less than 15 mm, and in the particular embodiment shown, cable 10 has this low outer diameter while including at least 72 optical fibers located in 6 buffer tubes.

Referring to FIG. 4, an optical fiber cable 60 is shown according to an exemplary embodiment. Cable 60 is substantially the same as cable 10 except as discussed herein. As shown, cable 60 includes at least 144 optical fibers, and includes at least 12 buffer tubes 22 each including 12 optical fibers 20. Like cable 10, cable 60 includes an outer diameter D3 that is less than 15 mm, and buffer tubes 22 are fully constrained buffer tubes 22′. In other embodiments, buffer tubes 22 of cable 60 may be buffer tubes 22″ and/or buffer tubes 22′″. In addition, cable 10 includes a jacketed central strength member 62 including central portion 64 and an outer jacket 66. In particular embodiments, central portion 64 may be relatively rigid, such as a metal material or glass reinforced plastic, and outer jacket 66 is a polymer coating.

Referring to FIG. 5, an optical fiber cable 70 is shown according to an exemplary embodiment. Cable 70 is substantially the same as cable 10 except as discussed herein. As shown, cable 70 includes at least 96 optical fibers, and includes at least 8 buffer tubes 22 each including 12 optical fibers 20. Like cable 10, cable 70 includes an outer diameter D3 that is less than 15 mm, and buffer tubes 22 are fully constrained buffer tubes 22′. In other embodiments, buffer tubes 22 of cable 70 may be buffer tubes 22″ and/or buffer tubes 22′″.

Referring to FIG. 6, an optical fiber cable 80 is shown according to an exemplary embodiment. Cable 80 is substantially the same as cable 10 except as discussed herein. As shown cable 80 includes at least 72 optical fibers, and includes at least 6 buffer tubes 22 each including 12 optical fibers 20. In the specific embodiment shown, buffer tubes 22 are unconstrained buffer tubes 22′″. Thus, cable 80 achieves very low bend loss through use of both unconstrained buffer tubes 22′″ which allows for fibers 20 to move during bending to achieve low strain positions in combination with the low diameter, low bend loss optical fibers discussed herein.

Referring to FIG. 7, an optical fiber cable 90 is shown according to an exemplary embodiment. Cable 90 is substantially the same as cable 10 except as discussed herein. As shown cable 90 includes a single buffer tube 92, and a stack 94 of a plurality of optical fiber ribbons 96. Each optical fiber ribbon 96 includes a plurality of optical fibers 20 surrounded by and supported by a polymeric matrix 98. In various embodiments, cable 90 includes at least four ribbons within stack 94 and each ribbon 96 supports four optical fibers 20. In some embodiments, like cable 10, cable 90 includes an outer diameter D3 that is less than 15 mm. In such embodiments cable 90 provides a low bend loss, low diameter ribbon cable utilizing the low diameter, low bend loss optical fibers discussed herein.

Low Bend Loss Optical Fibers

As noted above in various embodiments, optical fibers 20 discussed herein are configured to experience low signal loss during bending. In various embodiments, optical fibers 20 discussed above in relation to buffer tube and cable arrangements, may be any of the optical fibers discussed herein and specifically may be configured as the various embodiments of optical fiber 110, discussed below.

According to at least some embodiments the optical fibers have low bend-induced losses especially for tight bends, such as 15 mm diameter bends, for applications in data centers and fiber to the home installations. According to at least some embodiments the optical fibers disclosed herein are backward compatible with existing installed fiber network systems. According to at least some embodiments, the optical fibers disclosed herein have a 1310 mode field diameter of >9 microns in order to have low splice losses with existing installed optical fibers. Optical fibers disclosed herein can be cabled and can be part of a network system having a transmitter and receiver.

The “refractive index profile” is the relationship between refractive index or relative refractive index and the fiber radius. The radius for each segment of the refractive index profile is given by the abbreviations r₁, r₂, r₃, r_(4a), r₄, etc. and lower an upper case are used interchangeability herein (e.g., r₁ is equivalent to R₁).

The “relative refractive index percent” (also referred to herein as “refractive index delta percent”, “relative refractive index”, “refractive index delta”, and “relative refractive index delta”) is defined as Δ%=100×(n_(i) ²−n_(c) ²)/2n_(i) ², and as used herein n_(c) is the average refractive index of undoped silica. As used herein, the relative refractive index is represented by Δ and its values are given in units of “%”, unless otherwise specified. The terms: delta, Δ, Δ%, %Δ, delta %, % delta and percent delta may be used interchangeably herein. For embodiments described herein, an equivalent relative refractive index profile can be obtained by shifting the entire refractive index profile of the optical fiber is either up or down. In cases where the refractive index of a region is less than the average refractive index of undoped silica, the relative refractive index percent is negative and is referred to as having a depressed region or depressed index. In cases where the refractive index of a region is greater than the average refractive index of the cladding region, the relative refractive index percent is positive. An “updopant” is herein considered to be a dopant which has a propensity to raise the refractive index relative to pure undoped SiO₂. A “downdopant” is herein considered to be a dopant which has a propensity to lower the refractive index relative to pure undoped SiO₂. Examples of updopants include GeO₂ (germania), Al₂O₃, P₂O₅, TiO₂, Cl, Br. Examples of down dopants include fluorine and boron.

“Chromatic dispersion”, herein referred to as “dispersion” unless otherwise noted, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero. Zero dispersion wavelength is a wavelength at which the dispersion has a value of zero. Dispersion slope is the rate of change of dispersion with respect to wavelength.

“Effective area” is defined as in equation 1 as: A _(eff)=2π(∫f ² r dr)²/(∫f ⁴ r dr)   Eq. 1 where the integration limits are 0 to ∞, r is the radial distance from the center of the core, and f is the transverse component of the electric field associated with light propagated in the waveguide. As used herein, “effective area” or “A_(eff)” refers to optical effective area at a wavelength of 1550 nm unless otherwise noted.

The term “α-profile” refers to a refractive index profile, expressed in terms of Δ(r) which is in units of “%”, where r is radius, which follows the equation 2, shown below Δ(r)=Δ(r _(o))(1−[|r−r₀|/(r ₁ −r ₀)]^(α))   Eq. 2 where r₀ is the point (radial location) in the core at which Δ(r) is maximum, r₁ is the point at which Δ(r) % is zero, and r is in the range r_(i)≤r≤r_(f), where Δ is defined above, r_(i) is the initial point of the α-profile, r_(f) is the final point of the α-profile, and α is an exponent which is a real number (referred to as “core α”, “core alpha”, “alpha value” or “a value” herein).

The mode field diameter (MFD) is measured using the Peterman II method wherein, 2 w=MFD, and w²=(2∫f² r dr/∫[df/dr]² r dr), the integral limits being 0 to ∞.

The bend resistance of a waveguide fiber can be gauged by induced attenuation under prescribed test conditions, for example by deploying or wrapping the fiber around a mandrel of a prescribed diameter, e.g., by wrapping 1 turn around a either a 6 mm, 10 mm, or 20 mm or similar diameter mandrel (e.g. “1×10 mm diameter macrobend loss” or the “1×20 mm diameter macrobend loss”) and measuring the increase in attenuation per turn.

One type of bend test is the lateral load microbend test. In this so-called “lateral load” test (LLWM), a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 Newtons. A 70 Newton force is then applied to the plates and the increase in attenuation in dB/m is measured. The increase in attenuation is the lateral load attenuation of the waveguide in dB/m at a specified wavelength (typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm).

Another type of bend test is the wire mesh covered drum microbend test (WMCD). In this test, a 400 mm diameter aluminum drum is wrapped with wire mesh. The mesh is wrapped tightly without stretching, and should have no holes, dips, or damage. Wire mesh material specification: McMaster-Carr Supply Company (Cleveland, Ohio), part number 85385T106, corrosion-resistant type 304 stainless steel woven wire cloth, mesh per linear inch: 165×165, wire diameter: 0.0019″, width opening: 0.0041″, open area %: 44.0. A prescribed length (750 meters) of waveguide fiber is wound at 1 m/s on the wire mesh drum at 0.050 centimeter take-up pitch while applying 80 (+/−1) grams tension. The ends of the prescribed length of fiber are taped to maintain tension and there are no fiber crossovers. The attenuation of the optical fiber is measured at a specified wavelength (typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm); a reference attenuation is measured on the optical fiber wound on a smooth drum. The increase in attenuation is the wire mesh covered drum attenuation of the waveguide in dB/km at a specified wavelength (typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm).

Another type of bend test is the basketweave microbend loss test. In the basketweave microbend loss test, the fibers are wound at high tension on a glass spool and exposed to a temperature cycle. The testing apparatus comprises of a fixed diameter silica drum. The drum surface is smooth. In this test, the drum diameter is 110 mm. The fiber is wound onto the glass drum with a winding tension of 70 grams, and a pitch of 2 mm (distance between adjacent wraps of fiber). Multiple layers of fiber are wrapped with this tension and pitch. The pitch angles are reversed with each layer wound. The crossover of the tensioned fibers from the adjacent layers creates the microbend mechanism. A fiber length of 2.5 km is used. The initial fiber attenuation measurement is performed at about 23°, at about 45% RH (relative humidity) with the fiber deployed in the basketweave configuration with 70 grams of tension. Initial attenuation loss measurements are made at wavelengths of 1310 nm, 1550 nm, and 1625 nm. An OTDR (optical time domain reflectometer) is used to acquire the attenuation loss data.

After the initial attenuation loss measurement at 23° C., the fiber is subjected to thermal cycling. In the thermal cycling, the fiber is first cooled from 23° C. to −60° C. at a rate of 1° C./min. The fiber is maintained at −60° C. for 20 hours and then heated at a rate of 1° C./min back to 23° C. The fiber is maintained at 23° C. for 2 hours, then heated to 70° C. at a rate of 1° C./min and maintained at 70° C. for 20 hours. The fiber is then cooled to 23° C. at a rate of 1° C./min and maintained at 23° C. for two hours. The fiber is then subjected to a second thermal cycle, which was identical to the first thermal cycle—i.e., it is cooled from 23° C. to −60° C., then heated back to 23° C., maintained at that temperature for 2 hours and then heated from 23° C. to 70° C., after which it is cooled back to 23° C. Finally, after maintaining the fiber at a temperature of 23° C. for two hours, after the second cycle, the fiber is once again cooled to −60° C. at a rate of 1° C./min, held at −60° C. for 20 hours, and then further cooled at a rate of 1° C./min to −60° C. The fiber is held at −60° C. for 20 hours, then heated at a rate of 1° C./min back to 23° C. and held at 23° C. for 2 hours. The thermal cycling is concluded at this point.

During the thermal cycling of the fiber, the attenuation loss of the fiber is measured continuously. The maximum attenuation loss over the two thermal cycles down to −60° C. is determined, and the difference between this maximum attenuation loss and the initial attenuation loss at 23° C. is reported herein, as the basketweave microbend loss of the fiber over the temperature range from −60° C. to 70° C. In the thermal cycle down to −60° C., the difference between the attenuation loss measured at −60° C. and the initial attenuation loss at 23° C. is reported herein as the basketweave microbend loss of the fiber over the temperature range from −60° C. to 23° C.

The “pin array” bend test is used to compare relative resistance of waveguide fiber to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two measured attenuations. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. During testing, sufficient tension is applied to make the waveguide fiber conform to a portion of the pin surface. The increase in attenuation is the pin array attenuation in dB of the waveguide at a specified wavelength (typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm).

The theoretical fiber cutoff wavelength, or “theoretical fiber cutoff”, or “theoretical cutoff”, for a given mode, is the wavelength above which guided light cannot propagate in that mode. A mathematical definition can be found in Single Mode Fiber Optics, Jeunhomme, pp. 39-44, Marcel Dekker, New York, 1990 wherein the theoretical fiber cutoff is described as the wavelength at which the mode propagation constant becomes equal to the plane wave propagation constant in the outer cladding. This theoretical wavelength is appropriate for an infinitely long, perfectly straight fiber that has no diameter variations.

Fiber cutoff is measured by the standard 2 m fiber cutoff test, FOTP-80 (EIA-TIA-455-80), to yield the “fiber cutoff wavelength”, also known as the “2 m fiber cutoff” or “measured cutoff”. The FOTP-80 standard test is performed to either strip out the higher order modes using a controlled amount of bending, or to normalize the spectral response of the fiber to that of a multimode fiber.

By cabled cutoff wavelength, “cable cutoff”, “cable cutoff wavelength”, or “cabled cutoff” as used herein, we mean the cable cutoff wavelength determined by the 22 m cabled cutoff test described in the EIA-445 Fiber Optic Test Procedures, which are part of the EIA-TIA Fiber Optics Standards, that is, the Electronics Industry Alliance-Telecommunications Industry Association Fiber Optics Standards,

Unless otherwise noted herein, optical properties (such as dispersion, dispersion slope, etc.) are reported for the LP01 mode.

Optical fibers disclosed herein are capable of exhibiting an effective area at 1550 nm which is greater than about 70 microns², in some embodiments between 75 and 95 microns², for example between about 80 and 90 microns². In some embodiments, the optical mode effective area at 1550 nm is between about 82 and 88 microns².

The embodiments of the fiber 110 is (see, for example, FIG. 8) include a core 112 with comprising maximum refractive index delta percent Δ₁, and a cladding 120 that surrounds the core 112. In at least some embodiments disclosed herein the core alpha is larger than 5 (i.e., α>5). According to the exemplary embodiments described herein, fiber 110 is preferably a single mode fiber.

The cladding 120 includes inner cladding region 121 that is in contact with and surrounds the core 112, a depressed index cladding region 122 (also referred to as a trench region herein) that surrounds the inner cladding region 121. The cladding region 122 has a refractive index delta percent Δ₃. The outer cladding region 124 surrounds the trench region 122 and comprises a refractive index delta percent Δ₄. A trench region is a low refractive index region, surrounded by the higher refractive index regions. As shown for example, in FIG. 8, the trench region 122 within the cladding 120 is surrounded by two higher index cladding regions—i.e., cladding regions 121 and 124.

In the embodiments described herein Δ_(1max)>Δ₄; Δ₃<Δ₂; and Δ₄>Δ₃. In the embodiments illustrated in FIGS. 8-14, cladding regions 121, 122 and 124 are immediately adjacent one another. However, this is not required, and alternatively, additional cladding regions may be employed.

The core 112 comprises an outer radius ri (which is defined as where a tangent line drawn through maximum slope of the refractive index delta percent of central core 112 crosses the zero delta line) that is between 2.75 and 6 microns, in some embodiments between about 3 and 5.75 microns, for example between 3.5 and 5.6 microns, and in some embodiments 4-5 microns. Core 112 exhibits a refractive index delta percent Δ₁, (relative to pure silica). For example the maximum refractive index delta of the core, Δ_(1max), may be 0 percent (if it is made of pure silica) to 0.65 percent, or between 0.15 and 0.5 percent, and in some embodiments between about 0.2 and 0.5 percent. In some embodiments Δ_(1max)≥0.38, for example 0.5≥Δ_(1max)≥0.38.

In some embodiments, the core 112 exhibits a core alpha (α) wherein, α is greater than 5, for example at least 10. In some embodiments the core alpha is 15 or greater. In some embodiments, the core 112 may comprise an alpha between about 10 and 100, for example in some embodiments the core alpha, α, may be between 15 and 100, and in some embodiments between 15 and 40. A refractive index profile of an exemplary fiber embodiment with α₁ of about 20 is shown, for example, in FIG. 8.

In the embodiments illustrated in FIG. 10-14, the inner cladding region 121 is adjacent to the core 112 and comprises inner radius r₁ and outer radius r₂. The inner cladding region 121 preferably exhibits a refractive index delta percent, Δ₂≤0.3 (relative to pure silica). As stated above, Δ₁≥Δ₂. In the exemplary embodiments described herein, 0.15%≤Δ_(1max)−Δ₂≤0.5%, for example 0.2%<Δ_(1max)−Δ₂<0.4%, or 0.25%<Δ_(1max)−Δ₂<0.35. In some embodiments Δ₂ is 0 to 0.3 percent, for example between about 0 and 0.25 percent, or between 0.1 and 0.2 percent. Alternatively, for example, if the core region 112 (also referred to as a core herein) is made of pure silica, the inner cladding region 121 is downdoped relative to pure silica, such that Δ_(1max)−Δ₂≤0.5 percent. The outer radius r₂ of the inner cladding region 121 is between 5 and 17 microns, in some embodiments between about 7 and 15 microns, for example 6 to 12 microns, or 6 to 10 microns. In some embodiments the ratio of r₂/r₁ is >1.2. In some embodiments the ratio of r₂/r₁ is ≥1.25, for example 1.25≤r₂/r₁≤2.5. In at least some of the exemplary embodiments described herein 1.6≤r₂/r₁≤2.4. In at least some of the exemplary embodiments described herein 1.8≤r₂/r₁≤2.35. The above values of the radius r₁, the difference between Δ_(1max) and Δ₂, and the r₂/r₁ ratio help the fibers have 1300 nm≤λ₀≤1324 nm and MFD between 8.2 microns and 9.5 microns at the 1310 nm wavelength.

The trench region 122 (also referred to as depressed index cladding region herein) surrounds the inner cladding region 121. The trench region 122 has a refractive index delta percent Δ₃ that is smaller than Δ₂. In some embodiments Δ₃ is −0.4%≤Δ₃≤0.1%. For example, in some embodiments the trench is formed of pure silica, and Δ₃ is 0. In some embodiments the relative refractive index delta percent in the trench region 122 is essentially flat, i.e. the difference between the relative refractive index delta percent at any two radii within the trench region 122 is less than 0.03%, and in some embodiments less than 0.01%. In other embodiments there can be fluctuations as a result of small profile design or process variations. In some embodiments, the trench region 122 comprises silica which is substantially undoped with either fluorine or germania, i.e., such that the region is essentially free of fluorine and germania, i.e., less than 0.1 wt. % F or GeO₂. In some embodiments, the trench region is pure silica, in other embodiments it is silica doped with fluorine, in order to make −0.4%≤Δ₃≤0.1%. In some embodiments, 0.35%≤Δ_(1max)−Δ₃≤0.65%.

The trench region 122 preferably exhibits a width W_(t) (wherein W_(t)=r_(3−r2)) between about 4 microns and 22 microns, in some embodiments, between 8 and 20 microns. In some embodiments, the outer radius r₃ of the trench region may be not less than 10 microns, for example greater than 12 microns and less than 27 microns, or about 14.5 microns to about 25.5 microns in order to achieve good bend performance and a cable cutoff of ≤1260 nm.

Outer cladding region 124 surrounds the trench region 122 and comprises refractive index delta percent 44 which is higher than the refractive index delta percent Δ₃ of the trench region 122, thereby forming a region which is an “updoped” outer cladding region 124 with respect to the depressed index cladding region 122, e.g., by adding an amount of dopant (such as germania or chlorine) sufficient to increase the refractive index of the outer cladding region. In some embodiments, there is no fluorine or other down dopants in the trench region 122, and the outer cladding region 124 comprises an updopant, for example chlorine. In some embodiments, the chlorine concentration in the outer cladding region 124 is ≥1 wt. %. In some other embodiments, the chlorine concentration in the outer cladding region 124 is >1.2 wt. %. In still other embodiments, the chlorine concentration in the outer cladding region 124 is ≥1.5 wt. %. In yet other embodiments, the chlorine concentration in the outer cladding region 124 is ≥2 wt. %.

Outer cladding region 124 comprises a higher refractive index than that of the trench region 122, and may, for example, have a refractive index delta percent 44 which is between 0.12% and 0.4%. In some embodiments 0.12%≤Δ₄−Δ₃≤0.4%, for example in some embodiments 0.12%<Δ₄−Δ₃<0.3%. In some embodiments, the outer cladding region 124 is comprised of a first outer cladding region 123 (with an outer radius r_(4a)) and a second outer cladding region 125 (with an outer radius r₄), wherein the first outer cladding region 123 has a chlorine concentration of ≥1.2 wt % from r₃ to 40 microns. In some embodiments first outer cladding region 123 has a chlorine concentration of ≥1.5 wt. % from r₃ to 40 microns, and in some embodiments, the first outer cladding region 123 has a chlorine concentration of ≥2 wt. % from r₃ to 40 microns.

In some embodiments, the second outer cladding region 125 has a higher viscosity than the first outer cladding layer. That is, the viscosity of the glass of the second outer cladding region 125 is greater than the viscosity of glass of the first outer cladding region 123. In this embodiment the second outer cladding region 125 is the tension absorbing layer. In some embodiments, the second outer cladding region 125 has a chlorine concentration of ≤0.5 wt. % from r_(4a) to r₄ (where r_(4a) is the outer radius of the high (e.g., ≥1.2 wt. % Cl) updoped region, as shown for example in FIGS. 9, and r₄ is the outer radius of the glass optical fiber, for example, 62.5 microns). In some embodiments the second outer cladding region 125 has a chlorine concentration of ≤0.25 wt. % from its inner radius r_(4a) to the outer radius r₄. In some embodiments the second outer cladding region has a chlorine concentration, [Cl], of 0.0 wt. % [Cl]≤0.2 wt. % from r_(4a) to r₄. In some embodiments, 40 microns≤r_(4a)≤55 microns, for example r_(4a) is at 40 microns, 45 microns, 50 microns, or 55 microns. Preferably, the higher index portion (compared trench region 122) of outer cladding region 124 extends at least to the point where the optical power which would be transmitted through the optical fiber is greater than or equal to 90% of the optical power transmitted, more preferably to the point where the optical power which would be transmitted through the optical fiber is greater than or equal to 95% of the optical power transmitted, and most preferably to the point where the optical power which would be transmitted through the optical fiber is greater than or equal to 98% of the optical power transmitted, which is preferred to achieve good bend performance and a cable cutoff of ≤1260 nm. In some embodiments, this is achieved by having the “updoped” third annular region (i.e., the first outer cladding region 123) extend at least to a radial point of about 30 microns. In some embodiments, the “updoped” third annular region 123 extends at least to a radial point of about 40 microns, with a fourth annular region (i.e., the second outer cladding region 125) comprising essentially of silica surrounding the third annular region. In some embodiments, the cladding 120 has an outer diameter of 2 times the maximum radius, R_(max), of about 125 micron. As shown in FIG. 9, the updoped region 123 (i.e., the first outer cladding region) of the outer cladding 124, has a refractive index delta percent of Δ₄ and the cladding region 125 (i.e., the second outer cladding region) has a refractive index delta percent of 45, and Δ₄>Δ₅.

The profile volume V₃ of the trench region 122, is calculated using Δ⁽⁴⁻³⁾(r)rdr between radius r₂ and r₃, and thus is defined in equation 3 as

$\begin{matrix} {V_{3} = {2{\int_{r\; 2}^{r\; 3}{{\Delta_{({4 - 3})}(r)}{rdr}}}}} & {{Eq}.\mspace{14mu} 3} \end{matrix}$

All volumes are in absolute magnitude (i.e., V₃=|V₃|). In order to achieve good bend performance, the volume V₃ of the trench region 122 is preferably greater than 30% Δmicron², and may be greater than 45% Δmicron², and in some embodiments is greater than 50% Δmicron², and in some embodiments may be greater than 55% Δmicron². In some embodiments the volume V₃ of the trench region 122 is 30% Δmicron² to 90% Δmicron², for example 40 to 80% Δmicron².

In the exemplary embodiments disclosed herein the core 112 has a positive refractive index throughout. The core region 112 comprises a maximum refractive index delta percent Δ_(1 max) occurring between r=0 and r=3 microns. In these embodiments Δ_(1 max) is between about 0.38% and about 0.5%.

The fibers are capable of exhibiting a bend loss of less than 0.5 dB/turn when wound upon on a 15 mm diameter mandrel for fibers with MAC numbers ≥7.25. In some embodiments, the optical fibers disclosed herein have a MAC number of ≥7.6 or even ≥7.7 and in some examples, 7.6≤MAC≤8, and a zero dispersion wavelength, λ₀ of 1324 nm≥λ₀≥1300 nm. As used herein, MAC number means mode field diameter at 1310 (nm) divided by 22 m cable cutoff wavelength (nm).

The fibers disclosed herein may be drawn from optical fiber preforms made using conventional manufacturing techniques and using known fiber draw methods and apparatus, for example as is disclosed in U.S. Pat. Nos. 7,565,820, 5,410,567, 7,832,675, 6,027,062, the specifications of which are hereby incorporated by reference.

Various exemplary embodiments will be further clarified by the following examples. It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the claims.

EXAMPLES

Table 1 below lists characteristics of fiber embodiments examples 1-3. These fiber embodiments have refractive index profiles as shown in FIGS. 10-12. In particular, set forth below for each example is the refractive index delta percent Δ₁, alpha 1 (α1), and outer radius r₁ of the core 112; refractive index delta percent Δ₂, and outer radius r₂ of the inner cladding region 121; and refractive index delta percent Δ₃, and outer radius r₃, as well as profile volume V₃ of the trench region 122, which is calculated between r₂ and r₃; refractive index delta percent Δ₄. Also set forth are chromatic dispersion and dispersion slope at 1310 nm, chromatic dispersion and dispersion slope at 1550 nm, mode field diameter at 1310 nm and 1550 nm, lateral load wire mesh microbend at 1550 nm, pin array macrobend at 1550 nm, zero dispersion wavelength (Lambda 0), 22 m cable cutoff, MAC number at 1310 nm, 1×15 mm diameter bend loss (bend loss when the fiber is turned once around a 15 mm diameter mandrel), and spectral attenuation at 1310 and 1550 nm.

TABLE 1 Parameter Ex 1 Ex 2 Ex 3 Δ1max (%) 0.47 0.47 0.45 r₁ (micron) 4.3 4.3 4.3 Region12 20 20 20 Core Alpha Δ2 (%) 0.15 0.15 0.15 Δ1 − Δ2 0.32 0.32 0.30 r₂ (micron) 10 9 8.1 r₂/r₁ 2.3 2.1 1.9 Δ3 (%) 0.00 0.00 0.00 Δ2 − Δ3 0.15 0.15 0.15 r₃(micron) 20 19.5 20 Δ4 (%) 0.15 0.15 0.15 r₄(micron) 62.5 62.5 62.5 Max chlorine 1.5 1.5 1.5 concentration in outer cladding region 124, weight % Δ4 − Δ3 0.15 0.15 0.15 V₃ 45 45 50 (% micron²) Dispersion at 4.9E−04 0.346 0.25 1310 nm (ps/nm/km) Dispersion 0.088 0.099 0.091 Slope at 1310 nm (ps/nm²/km) Lambda 1318 1315 1315 zero, nm Dispersion at 17.5 18 18 1550 nm (ps/nm/km) Dispersion at 0.062 0.062 0.063 Slope 1550 nm (ps/nm{circumflex over ( )}2/km) MFD at 1310 nm 9.2 9.16 9.25 (micron) MFD at 1550 nm 10.44 10.34 10.38 (micron) LLWM @ 0.6 0.56 0.77 1550 nm, dB/m WMCD at 0.04 0.04 0.04 1550 nm, dB/km Pin Array at 14.9 15.0 23.9 1550 nm, dB Cable Cutoff 1206 1206 1200 (nm) Aeff at 1310 nm 66.5 65.9 67.2 (micron²) Aeff at 1550 nm 85.6 84.0 84.6 (micron²) MAC # (MFD 7.63 7.60 7.71 at 1310 nm/ Cable Cutoff) 1 × 15 mm 0.19 0.2 0.29 diameter bend loss at 1550 nm (dB/turn) 1 × 20 mm 0.047 0.047 0.074 diameter bend loss at 1550 nm (dB/turn) 1 × 30 mm 0.0045 0.0045 0.01 diameter bend loss at 1550 nm (dB/turn) Attn at 1550 nm, 0.18 0.18 0.18 dB/km Attn at 1310 nm, 0.32 0.32 0.32 dB/km

As can be seen in Table 1 above, the exemplary fibers shown in Table 1 employ a glass core region 112 having index Δ₁, an inner cladding region 121 having index Δ₂, and cladding trench region 122 having refractive index delta percent Δ₃, and an outer cladding region 124 having refractive index delta percent Δ₄; wherein Δ_(1max)>Δ₂; Δ_(1max)>Δ₄; Δ₃≥Δ₂; Δ₄>Δ₃, wherein the difference between Δ_(1max) and Δ₂ is greater than or equal to at least 0.15, difference between Δ_(1max) and Δ₃ is greater than or equal to at least 0.35 (e.g., 0.38≤Δ_(1max)−Δ₃≤0.65); the difference between Δ₂ and Δ₃ is greater than or equal to at least 0.08 (e.g., 0.08≤Δ₂−Δ₂≤0.4); and the difference between Δ₄ and Δ₃ is greater than or equal to at least 0.08 (e.g., 0.1≤Δ₄−Δ₃≤0.4, or 0.1≤Δ₄−Δ₃≤0.03); and the absolute value of profile volume, |V₃| is at least 30% micron². These fibers have mode field diameters (MFD) at 1310 nm between 9 microns and 9.5 micron, for example between 9.2 microns and 9.5 microns and a zero dispersion wavelength between 1300 nm and 1324 nm.

Table 2 below lists characteristics of a fiber example 4 embodiment. This fiber embodiments has the refractive index profile as shown in FIG. 13.

TABLE 2 Parameter Ex 4 Δ1max (%) 0.53 r₁ (micron) 4.4 Region12 Core Alpha 20 Δ2 (%) 0.2 Δ1 − Δ2 0.33 r₂ (micron) 10 r₂/r₁ 2.27 Δ3 (%) 0.00 Δ2 − Δ3 0.2 r₃(micron) 18.2 Δ4 (%) 0.2 r_(4a)(micron) 45 Max chlorine concentration. in 2 outer cladding region 124, weight % Δ4 − Δ3 0.2 Δ5 (%) 0 r₄ (micron) 62.5 V₃ (% micron2) 46.2 Dispersion at 1310 nm 0.483 (ps/nm/km) Dispersion Slope at 1310 nm 0.089 (ps/nm²/km) Lambda zero, nm 1312 Dispersion at 1550 nm 18.1 (ps/nm/km) Dispersion at Slope 1550 nm 0.062 (ps/nm²/km) MFD at 1310 nm (micron) 9.16 MFD at 1550 nm (micron) 10.31 LLWM @ 1550 nm, dB/m 0.4 WMCD at 1550 nm, dB/km 0.04 Pin Array at 1550 nm, dB 8.96 Cable Cutoff (nm) 1257 Aeff at 1310 nm (micron²) 66.1 Aeff at 1550 nm (micron²) 81.7 MAC # (MFD at 1310 nm/ 7.29 Cable Cutoff) 1 × 15 mm diameter bend loss 0.102 at 1550 nm (dB/turn) 1 × 20 mm diameter bend loss 0.023 at 1550 nm (dB/turn) 1 × 30 mm diameter bend loss 0.002 at 1550 nm (dB/turn) Attn at 1550 nm, dB/km 0.18 Attn at 1310 nm, dB/km 0.32

As can be seen in Table 2 above, the exemplary fibers such as that depicted in FIG. 9 and FIG. 13 employ a glass core region 112 having refractive index delta percent Δ_(1max), an inner cladding region 121 having refractive index delta percent Δ₂, and trench region 122 having refractive index delta percent Δ₃, and an first outer cladding region 123 having refractive index delta percent Δ₄ and a second outer cladding region 125 having a refractive index delta percent Δ₅; wherein Δ_(1max)>Δ₂; Δ_(1max)>Δ₄; Δ₃>Δ₂; Δ₄>Δ₃, wherein the difference between Δ_(1max) and Δ₂ is greater than or equal to at least 0.15, difference between Δ_(1max) and Δ₃ is greater than or equal to at least 0.35 (e.g., 0.38≥Δ_(1max)−Δ₃≤0.65); the difference between Δ₂ and Δ₃ is greater than or equal to at least 0.08 (e.g., 0.08≤Δ₂−Δ₂≤0.4); and the difference between Δ₄ and Δ₃ is greater than or equal to at least 0.08 (e.g., 0.1≤Δ₄−Δ₃≤0.4, or 0.1≤Δ₄−Δ₃≤0.3); and an absolute value of profile volume, |V₃| of at least 30% micron². In this embodiment, the cladding region 125 is a silica layer with a relative refractive index percent of about zero. The cladding region 125 (i.e., the second outer cladding region) acts as a stiff tension absorbing layer. This fiber embodiment has a mode field diameter (MFD) at 1310 nm between 9 microns and 9.5 micron, and a zero dispersion wavelength between 1300 nm and 1324 nm.

The fiber embodiments described herein exhibit a cable cutoff less than or equal to 1260 nm and a bend loss of less than 0.5 dB/turn when wound upon on a 15 mm diameter mandrel. These fibers also exhibit a mode field diameter between about 9 and 9.5 microns at 1310 nm, a zero dispersion wavelength between 1300 and 1324 nm, a dispersion slope at 1310 nm which is less than or equal to 0.092 ps/nm²/km. These fibers exhibit a Wire Mesh Covered Drum (WMCD) bend loss at 1550 nm which is less than or equal to 0.07 dB/km, in some embodiments less than or equal to 0.06 dB/km, and in some embodiments less than or equal to 0.05 dB/km. These fibers also exhibit a pin array bend loss at 1550 nm which is less than 8.5 dB, in some embodiments less than 5 dB and in some embodiments less than 4 dB. These fibers exhibit a Basketweave microbend loss at 1550 nm which is less than or equal to 0.05 dB/km, in some embodiments less than or equal to 0.025 dB/km, and in some embodiments less than or equal to 0.01 dB/km.

Many of these fibers also exhibit a bend loss at 1550 nm, when wound upon on a 15 mm diameter mandrel, of less than 0.5 dB/turn, and in some cases less than 0.2 dB/turn. These fibers also exhibit a bend loss at 1550 nm, when wound upon on a 20 mm diameter mandrel, of less than 0.2 dB/turn, in some embodiments less than 0.15 dB/turn, and some fibers in some embodiments less than 0.1 dB/turn. These fibers also exhibit a bend loss at 1550 nm, when wound upon on a 30 mm diameter mandrel, of less than 0.02 dB/turn, for example less than 0.005 dB/turn, or even less than 0.003 dB/turn.

Such bend loss and attenuation performance numbers are attainable using a primary and secondary coating applied to the fiber, wherein the Young's modulus of the primary is less than 2 MPa, in some embodiments less than 1 MPa, and in some embodiments less than 0.5 MPa. The Young's modulus of the secondary coating is greater than 500 MPa, in some embodiments greater than 1000 MPa, and in some embodiments greater than 1500 MPa. In some embodiments, the outer diameter of the secondary coating is 242 microns. In some other embodiments, the outer diameter of the secondary coating is 200 microns.

Table 3 provides data of a manufactured optical fiber embodiment (Example 5 fiber). The refractive index profile of optical fiber example 5 fiber is illustrated in FIG. 14.

TABLE 3 Data for manufactured optical fiber Parameter Ex 5 Δ_(1max) (%) 0.48 r₁ (microns) 4.87 Core Alpha, α 20 r₂ (microns) 6.11 r₂/r₁ 1.25 Δ₂ (%) 0.153 R₃ (microns) 19.8 Δ₃ (%) 0 V₃ (%Δ micron²) 60 Δ₄ (%) 0.168 Chlorine conc. in outer cladding region 124, 1.7 weight % r₄ (microns) 62.5 Dispersion at 1310 nm 0.565 (ps/nm/km) Dispersion Slope at 1310 nm 0.091 (ps/nm²/km) Dispersion at 1550 nm 18.1 (ps/nm/km) Zero Dispersion Wavelength (nm) 1304 MFD at 1310 nm (microns) 9.34 MFD at 1550 nm (microns) 10.45 Aeff at 1550 nm (micron²) 85.8 Cable Cutoff (nm) 1204 Macrobend Loss for 15 mm 0.078 mandrel diameter at 1550 nm (dB/turn) Macrobend Loss for 20 mm 0.084 mandrel diameter at 1550 nm (dB/turn) Macrobend Loss for 30 mm 0.005 mandrel diameter at 1550 nm (dB/turn) Microbend loss at 1550 nm in 0.005 Basket-weave test at −60 C. for 242 microns coating diameter (dB/km) Microbend loss at 1550 nm in 0.03 Basket-weave test at −60 C. for 200 microns coating diameter (dB/km) Microbend loss at 1550 nm in 0.03 Basket-weave test at −60 C. for 200 microns coating diameter (dB/km)

In the embodiment of Table 3, the optical fibers exhibits a basketweave microbend loss at −60° C. at 1550 nm which is less than or equal to 0.05 dB/km, for example less than or equal to 0.03 dB/km.

In some embodiments, the fiber core may comprise a relative refractive index profile having a so-called centerline dip which may occur as a result of one or more optical fiber manufacturing techniques. However, the centerline dip in any of the refractive index profiles disclosed herein is optional.

The optical fiber disclosed herein comprises a core 112 and a cladding 120 surrounding and directly adjacent to the core. According to some embodiments, the core is comprised of silica doped with germanium, i.e. germania doped silica. Dopants other than germanium, singly or in combination, may be employed within the core, and particularly at or near the centerline, of the optical fiber disclosed herein to obtain the desired refractive index and density. In embodiments, the core region 112 of the optical fiber 110 disclosed herein has a non-negative refractive index profile, more preferably a positive refractive index profile, with the inner cladding region 121 surrounding and directly adjacent to core region 112.

In various embodiments discussed herein, the optical fibers include one or more protective layer (e.g., polymer layers) located outside of and surrounding outer cladding region 124, and in at least some embodiments, these protective layers are configured to provide puncture resistance to the optical fiber. For example, the optical fiber disclosed herein may be surrounded by a protective coating, e.g. a primary coating P contacting and surrounding the outer cladding region 124. In various embodiments, the primary coating P has a Young's modulus of less than 1.0 MPa, in some embodiments, less than 0.9 MPa, and in some embodiments not more than 0.8 MPa. In various embodiments, the optical fibers discussed herein further includes a secondary coating S contacting and surrounding the primary coating P. In various embodiments, the secondary coating S has a Young's modulus of greater than 1200 MPa, and in some embodiments greater than 1400 MPa. In some embodiments, optical fibers discussed herein include a primary coating P have intrinsic modulus of elasticity less than 0.5 MPa, specifically less than 0.2 MPa and even more preferably less than 0.15 MPa, while glass transition temperature is between −25 and −35C, and in some such embodiments, the diameter of the primary coating is preferably less than 165 um, specifically less than 160 um and even more specifically less than 150 um, and in such embodiments, the secondary coating diameter is less than 250 microns and more specifically is less than 210 microns. In various embodiments, the secondary coating has a modulus of elasticity larger than 1200 MPa, specifically larger than 1500 MPa and more specifically larger than 1800 MPa. In particular embodiments, reduced diameter optical fibers discussed herein have secondary coatings with modulus of elasticity of larger than 1700 MPa have a puncture resistance load of larger than 25 g, as shown in Table 4 below. The test method for the puncture resistance of the optical fiber coating can be found in 52^(nd) IWCS (International Wire and Cable Symposium) Proceedings, p. 237-245.

TABLE 4 Puncture resistance testing for reduced diameter optical fibers Secondary coating cross- Fiber sectional area, microns² Minimal puncture load, grams 1 9450 28.0 2 10912 26.8 3 11306 28.2

According to some embodiments, with primary and secondary coatings, the outer diameter of the secondary coating is less than 250 microns. According to some embodiments the fiber further is coated with primary and secondary coatings, and the outer diameter of the secondary coating is less than 210 microns.

As used herein, the Young's modulus, elongation to break, and tensile strength of a cured polymeric material of a primary coating is measured using a tensile testing instrument (e.g., a Sintech MTS Tensile Tester, or an INSTRON Universal Material Test System) on a sample of a material shaped as a film between about 0.003″ (76 micron) and 0.004″ (102 micron) in thickness and about 1.3 cm in width, with a gauge length of 5.1 cm, and a test speed of 2.5 cm/min.

Additional description of suitable primary and secondary coatings can be found in PCT Publication WO2005/010589 which is incorporated herein by reference in its entirety.

Preferably, the optical fibers disclosed herein have a low OH content, and preferably have an attenuation curve which exhibits a relatively low, or no, water peak in a particular wavelength region, especially in the E-band. The optical fiber disclosed herein preferably has an optical attenuation (spectral) at 1383 nm which is not more than 0.10 dB/km above an optical attenuation at 1310 nm, and more preferably not more than the optical attenuation at 1310 nm. The optical fiber disclosed herein preferably has a maximum hydrogen induced attenuation change of less than 0.03 dB/km at 1383 nm after being subjected to a hydrogen atmosphere, for example 0.01 atm partial pressure hydrogen for at least 144 hours.

A low water peak generally provides lower attenuation losses, particularly for transmission signals between about 1340 nm and about 1470 nm. Furthermore, a low water peak also affords improved pump efficiency of a pump light emitting device which is optically coupled to the optical fiber, such as a Raman pump or Raman amplifier which may operate at one or more pump wavelengths. Preferably, a Raman amplifier pumps at one or more wavelengths which are about 100 nm lower than any desired operating wavelength or wavelength region. For example, an optical fiber carrying an operating signal at wavelength of around 1550 nm may be pumped with a Raman amplifier at a pump wavelength of around 1450 nm. Thus, the lower fiber attenuation in the wavelength region from about 1400 nm to about 1500 nm would tend to decrease the pump attenuation and increase the pump efficiency, e.g. gain per mW of pump power, especially for pump wavelengths around 1400 nm.

The fibers disclosed herein exhibit low PMD values particularly when fabricated with OVD processes. Spinning of the optical fiber may also lower PMD values for the fiber disclosed herein.

Cable examples

Table 5 shows modeled results for optical fiber cable designs having fibers disclosed herein. Shown in the table below, these exemplary embodiments include buffer tube diameter, buffer tube wall thickness, number of optical fibers in each buffer tube, the diameter of the optical fiber including the coating and coloring layers, the overall number of optical fibers in the cable, the number of buffer tubes in the cable, the central member minimum diameter (including the strength elements and upjacketing), the minimum cable core diameter, the fiber density in the cable core and the Ω parameter (fiber diameter/inside diameter of the buffer tube). Optical fiber cables in these examples can include stranding of the buffer tubes, stranding binder yarns and/or thin film binder to hold the buffer tubes, additional strength members outside the cable core, armor, and cable jacketing.

TABLE 5 Optical Fiber Cables Example Example Example Example Example Example Example Parameter 1 2 3 4 5 6 7 Buffer Tube Inside 0.92 0.77 0.89 0.89 0.84 0.84 0.84 Diameter, mm Buffer Tube 0.15 0.15 0.15 0.15 0.15 0.10 0.05 Thickness, mm Number of Fibers in 8 8 12 12 12 12 12 buffer Tube Coated + Colored 0.25 0.21 0.21 0.21 0.21 0.21 0.21 Fiber Diameter, mm Overall fiber count in 96 96 72 144 144 144 144 Cable Buffer tubes in cable 12 12 6 12 12 12 12 Min central member 3.48 3.06 1.19 3.41 3.28 2.99 2.71 diameter, mm Min Cable Core 5.92 5.20 3.57 5.79 5.56 5.07 4.59 Diameter, mm Fiber density in Cable 3.49 4.51 7.19 5.47 5.93 7.12 8.72 Core (N/mm²) Omega 3.68 3.67 4.24 4.24 4.00 4.00 4.00 Example Example Example Example Example Example Example Parameter 8 9 10 11 12 13 14 Buffer Tube Inside 1.20 1.22 1.22 0.99 0.87 1.22 1.22 Diameter, mm Buffer Tube 0.15 0.15 0.10 0.10 0.15 0.15 0.15 Thickness, mm Number of Fibers in 24 24 24 24 24 24 24 buffer Tube Coated + Colored 0.21 0.21 0.21 0.17 0.15 0.21 0.21 Fiber Diameter, mm Overall fiber count in 144 144 144 144 144 288 288 Cable Buffer tubes in cable 6 6 6 6 6 12 12 Min central member 1.50 1.53 1.43 1.19 0.88 4.37 4.37 diameter, mm Min Cable Core 4.50 4.57 4.27 3.57 3.22 7.41 7.41 Diameter, mm Fiber density in Cable 9.05 8.77 10.04 14.36 17.64 6.67 6.67 Core (N/mm²) Omega 5.71 5.81 5.81 5.82 5.80 5.81 5.81

The cable examples in Table 5 show cables comprising buffer tube inner diameters between 0.75 to 1.25 mm, buffer tube wall thicknesses of 0.05 to 0.15 mm, the number of optical fibers in each buffer tube from 8 to 24, the diameter of the optical fiber including the coating and coloring layers from 0.21 to 0.25 mm, the overall number of optical fibers in the cable from 72 to 288, the number of buffer tubes in the cable from 6 to 12, the central member minimum diameter, including the strength elements and upjacketing (e.g., polymer coating on the strength elements,) from 0.88 mm to 4.37 mm, the minimum cable core diameter from 3.22 mm and 7.41 mm, the fiber density in the cable core from 3.49/mm² and 17.64/mm², and the Ω parameter (fiber diameter/inside diameter of the buffer tube) from 3.67 and 5.81. The examples herein show that the diameter ratio parameter Ω ranges from 2.25+0.143(N)≤Ω≤1.14+0.313(N) and in some preferred embodiments ranges from 2.25+0.143(N)≤Ω≤2.66+0.134(N).

It is to be understood that the foregoing description is exemplary only and is intended to provide an overview for the understanding of the nature and character of the fibers which are defined by the claims. The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments which, together with their description, serve to explain the principals and operation. It will become apparent to those skilled in the art that various modifications to the embodiments as described herein can be made without departing from the spirit or scope of the appended claims.

While the specific cable embodiments discussed herein and shown in the figures relate primarily to cables that have a substantially circular cross-sectional shape defining a substantially cylindrical internal bore, in other embodiments, the cables discussed herein may have any number of cross-section shapes. For example, in various embodiments, cable jacket 12 may have an oval, elliptical, square, rectangular, triangular or other cross-sectional shape. In such embodiments, the passage or lumen of the cable may be the same shape or different shape than the shape of cable jacket 12. In some embodiments, cable jacket 12 may define more than one channel or passage. In such embodiments, the multiple channels may be of the same size and shape as each other or may each have different sizes or shapes.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred. In addition, as used herein the article “a” is intended to include one or more than one component or element, and is not intended to be construed as meaning only one.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosed embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the embodiments may occur to persons skilled in the art, the disclosed embodiments should be construed to include everything within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A densely packed, low bend loss optical fiber cable comprising: an outer cable jacket; a plurality of buffer tubes surrounded by the cable jacket, each buffer tube includes an inner surface defining a channel having a diameter, D1, and an outer surface facing an inner surface of the cable jacket; and a plural number, N, of optical fibers, located within the channel of each buffer tube and surrounded by the inner surface of the buffer tube, wherein each optical fiber has an outer diameter, D2; wherein the N optical fibers are densely packed within each buffer tube such that a diameter ratio parameter, Ω, is defined as 2.25+0.143(N)≤Ω≤1.14+0.313(N); wherein each of the plural number of optical fibers comprise: a mode field diameter of larger than 9 microns at 1310 nm; a cable cutoff of less than 1260 nm; a macrobend loss of less than 0.5 dB/turn at 1550 nm for a mandrel diameter of 15 mm; and wherein the plural number of optical fibers are arranged into an outer group and an inner group within each buffer tube, wherein the outer group comprises a plurality of optical fibers and the inner group comprises at least two optical fibers, wherein the optical fibers of the outer group and of the inner group are packed within the buffer tube such that a maximum gap length measured between any pair of adjacent optical fibers of the outer group is less than the outer diameters of the at least two optical fibers of the inner group such that the at least two optical fibers of the inner group are blocked from moving from the inner group to the outer group within the buffer tube.
 2. The optical fiber cable of claim 1, wherein each of the plural number of optical fibers further comprises an outer diameter of less than 210 microns.
 3. The optical fiber cable of claim 1, wherein the plurality of buffer tubes is at least four, and wherein the plural number N of optical fibers within each buffer tube is at least four.
 4. The optical fiber cable of claim 3, wherein the plural number N of optical fibers within each buffer tube is 12, wherein the outer cable jacket has an outer diameter less than 15 mm, and the outer diameter of each optical fiber is less than 210 microns.
 5. The optical fiber cable of claim 1, wherein N is greater than
 6. 6. The optical fiber cable of claim 5, wherein the N optical fibers are densely packed within each buffer tube such that Ω≤2.66+0.134(N).
 7. The optical fiber cable of claim 1, wherein N is greater than
 24. 8. The optical fiber cable of claim 1, wherein each buffer tube has a wall thickness between 50 microns and 250 microns.
 9. The optical fiber cable of claim 1, wherein each buffer tube is formed from a relatively rigid polymer material having a modulus of elasticity at 25° C. between 0.8 GPa and 3 GPa.
 10. The optical fiber cable of claim 1, wherein a circle circumscribing the plural number N of optical fibers, has a diameter D4, wherein the ratio D4/D1 is greater than 0.95.
 11. The optical fiber cable of claim 1, wherein each of the plural number N of optical fibers is coated with primary and secondary coatings, wherein the outer diameter of the secondary coating is less than 210 microns, wherein the modulus of elasticity of the primary coating is less than 0.5 MPa and the modulus of elasticity of the secondary coating is greater than 1200 MPa such that a coating puncture load of the primary and secondary coating is greater than 25 g. 